Brillouin flow gun



Filed April 10 1959 6% Mb hm vb ATTORNEY United States Patent 2,997,615BRILLOUIN FLOW GUN Robert Adler, Northfield, Ill., assignor to ZenithRadio Corporation, a corporation of Delaware Filed Apr. 10, 1959, Ser.No. 805,473 8 Claims. (Cl. 313-84) The present invention relates to anelectron gun. More particularly, it has to do with an electron gun fordeveloping an electron beam of accurately controlled constant radius.

Many electron-beam devices require a pencil-1ike electron beam for theiroperation. It is especially desirable in certain forms of traveling-wavetubes, electron-beam parametric amplifiers and the like to utilize abeam of small diameter and of constant radius. Such beams have in thepast been obtained by the use of often impractically strong focusingfields, axially of the beam fiow. By further achieving what is known asBrillouin flow, a beam of constant radius has been produced with alesser, more practical strength of focusing field required.

Brillouin flow is a condition in which the beam, if viewable, wouldresemble a solid rod moving end-wise and rotating about its principalaxis, all electrons moving with the same forward velocity along thepath. For this condition, the focusing field has a strength justsufficient to balance out the space charge and centrifugal forcestending to diverge the beam.

The classic approach to achieving Brillouin flow has involved criticalmagnetic shielding, initially producing a beam having no rotationalvelocity, and then introducing the beam into a magnetic field at ahighly critical angle related to the field configuration. After properintroduction of the beam into the field, it is converged to theequilibrium radius (at which inward and outward forces acting on theelectrons are in balance) and steps are simultaneously taken toeliminate any radial motion components existing at the point equilibriumis obtained. All this requires high precision of manufacture, complexand bulky equipment, and most careful adjustment.

In principle, Brillouin flow may be obtained with a point source ofelectrons located within the focusing field. With this approach, thedifiicult problems associated with entry of a beam into a focusing fieldare avoided. One suggestion in this general direction has been toprovide a cathode structure productive of an electron beam of very smalldiameter and high density. After passing through an accelerating field,the beam is subjected to a decelerating field in which it is allowed toexpand under the influence of space charge forces, while a magneticfield imparts rotational movement to the electrons. Subsequently, thebeam is passed through a focusing anode Where components of radialmotion are reduced, theoretically resulting in a beam in which there isno radial movement. However, this telchnique requires critical designand adjustment. Moreover, this technique is applicable only where spacecharge forces are sufficiently large, and even then considerable axialdistance is required to permit the space-charge forces to expand thebeam to the desired equilibrium radius. In addition, no currentadjustment is possible since this would upset the space charge forces;the focusing anode is highly critical as to its potential and locationand any misadjustment in this respect leaves an undesired scallop on thebeam throughout its travel.

It is therefore a principal object of the present inventionto provide anew and improved electron beam of constant radius and which overcomesone or more of the aforenoted deficiencies and difficulties.

Another object of the present invention is to provide a new and improvedelectron gun for generating a beam of constant radius while requiring aminimum of space.

A further object of the present invention is to provide a new andimproved electron gun for generating a beam of constant radius and whichgun is adjustable over a wide range of desired beam currents and ischaracterized by comparative ease of adjustment and simplicity ofoperation.

In carrying out the invention, an electron beam of predeterminedcross-sectional area is projected along a path and subjected to amagnetic field, in which the beam originates, directed along the pathand of predetermined strength. The beam is first accelerated, thenstrongly constricted after which it expands to a larger cross-sectionalarea at which the net radial forces acting on electrons in the beamapproach zero. The expansion is then gradually tapered off and theelectrons are directed parallel to the path with substantially zeroradial motion.

An electron gun constructed in accordance with the invention includes acathode for producing an electron stream of predeterminedcross-sectional area. Following the cathode is an eccelerating regionterminated by a strongly convergent electrostatic lens. This lensincludes a first anode, adjacent and positive with respect to thecathode, having an aperture generally centered with respect to thestream and substantially smaller than the latter. Next along the streamis a second apertured anode substantially positive with respect to thefirst anode. Subsequently, the gun includes a convergent electrostaticlens system including a third apertured anode its aperture substantiallylarger than that of said first anode.

The features of the invention which are [believed to be novel are setforth with particularity in the appended claims. The organization andmanner of operation of the invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like elements are identified by like numerals in each of thefigures, and in which:

FIGURE 1 is a schematic diagram of an electron beam system employing oneembodiment of the present invention;

FIGURE 2 is a schematic diagram of an electron beam system employinganother embodiment of the present invention; and

FIGURE 3 is a longitudinal cross-seetional'view of an electron-beamamplifier tube employing an electron gun constructed in accordance withthe present invention.

As illustrated in FIGURE 1, the electron gun includes a cylindricalcathode 10, one end of which is coated with an emissive material 11.Cathode 10 is oriented to project electrons generally in a directionalong an electron beam path 12. Spaced a very short distance in front ofcathode =10 is a first anode 13 having an aperture 14 centered on axisll. Aperture 14 has a cross-sectional area substantially smaller thanthat of the electron stream emitted from cathode 10.

Next beyond anode 13 is a second anode 15 having an aperture 16 centeredon axis 12. Following anode 15are third and fourth anodes 17 and 18successively spaced along the beam path and having apertures 19 and 20,respectively.

As indicated in FIGURE 1, the electron beam is eventually intercepted bya collector 21 disposed transversely of axis 12. In the presentembodiment, the entire electron beam path is subjected to a magneticfield H having flux lines flowing parallel to axis 12 as schematicallyindicated.

In operation, an electron stream having an effective envelope asindicated at 25, with a radius R, is projected toward and substantiallyintercepted by first anode '13 which is at a potential positive withrespect to cathode 10; anode 13 is coupled to a source A+. Anode 13 isspaced but a short distance from cathode 10. Only a small portion of thebeam passes through aperture 14, having an envelope 27. Second anode 15is at a potential substantially higher than that of first anode 13 andtherefore serves as an accelerator; to this end anode 15 is connected toa source B+.

The combination of cathode 11 and anodes 13 and 15 creates a convergentelectrostatic lens 26 effectively located at aperture 14 and acting onthe beam ahead of anode 15. Convergent lens 26 serves to highlyconstrict the beam as indicated at point 28, though in practice notactually to a point. Because of the high potential on anode 15, theconstriction of the beam is rather violent; after having reached itssmallest diameter, the beam diverges quite rapidly.

Anodes 17 and 18, the apertures of which are larger than aperture 14,are at potentials positive with respect to the cathode but substantiallysmaller than the potential on accelerating anode 15; for this purpose,anodes 17 and 18 are connected to sources C[ and Dj, re-

spectively. The combination of anodes 13, 15 and 17 constitutes adivergent electrostatic lens 29 effectively located at aperture 16.Because the beam passes through its smallest cross-section in theimmediate vicinity of aperture 16, the lens located at that point exertscomparatively little influence on its path. After passing aperture 16,the stream continues to expand or spread apart so that the electrondensity is reduced below that in stream 25.

Electrodes 15, 17 and 18 constitute a gradually increasing convergentelectrostatic lens system 30 effectively located at aperture 19. Thislens system is of a strength and is spaced from anode 15 by a distancesufiicient to permit the electrons in the beam to spread nearly to thefinal desired beam radius r, but no more, and to effect a gradualtapering-off of the divergent action and straighten out the beamenvelope to a position parallel with axis 12.

Adjustment of the parallelizing of the envelope is preferably enhancedby the use of a further convergent electrostatic lens of which anodes 17and 18 and the next following electrode (FIGURE 2 or 3) form a part. Tothis end, electrode 18 is at a potential intermediate the potentials onanode 17 and the next electrode. Thus, by adjusting the potential ofelectrode 18,

it is possible to effectively shift some of the total refractive powerof apertures 19 and 20 at will from one of these to the other, thusgaining an additional measure of control over the final diameter of thebeam.

employing the dual-electrode convergent lens following accelerator anode15, two independent variables are available for adjustment. This enablesmost accurate operation while at the same time greatly reducing theprecision of mechanical design and manufacture required.

In a typical practical and successful construction, the

.following dimensions are utilized; they are set forth merely forpurposes of illustration since it will be appreciated that manydifferent structures may be employed as the requisite electron lens.Cathode has a diameter of 0.132. inch and its emissive coating is spacedfrom anode 13 by a distance of .015 inch. Anode 13 has a thickness of0.005 inch and its aperture has a diameter of 0.012 inch. Anodes 15, 17and 18 are spaced successively 4 along the beam path at intervals of0.030 inch and have apertures of 0.030 inch. Collector 21 is spaced fromanode 18 by a distance of 3.3 centimeters. This particular gun whenoperated has been found to produce a beam of constant radius atelectrode potentials (with respect to the cathode) on anodes 13, 15, 17and 18 of about 8, 150, 15 and 7 volts, respectively; accordingly, theaccelerator electrode is biased at a potential at least an order ofmagnitude greater than are the other lens electrodes. Collector 21 isbiased with a potential of about 6 volts. In one tube in which this gunis employed, the strength of magnetic field H is approximately 200gauss; the beam rotates at a rate well below the cyclotron frequency atthis field strength. With any particular construction the exact voltagesare best determined by a small amount of experimentation to obtainmaximum beam transmission with minimum interception by the electrodes ofthe gun and other elements in the tube, of course excluding anode 13 andcollector 21.

In some instances it has been found desirable to eliminate what appearto be a few stray or marginal electrons existing around the envelopesurface. To this end a collimating system is added to the electron gunsystem, as indicated in FIGURE 2. This system includes at least one andpreferably a pair of beam-slicer anodes 32 and 33 spaced along the beampath from anode 18. Slicer anodes 32 and 33 have apertures 34 and 35,respectively, centered on axis 12 and of a diameter slightly less thanthat of aperture 20 in anode 18; for example, in a gun otherwise likethat of FIGURE 1 apertures 34 and 35 each have a diameter of 0.016 inch.In operation, anodes 32 and 33 are biased at a potential usuallyintermediate that of the nearest adjacent other electrodes. Anodes 32and 33 act to intercept any such marginal or stray electrons whileaccurately defining the beam envelope surface. Otherwise, the electrongun depicted in FIGURE 2 is identical with that described above withrespect to FIGURE 1.

A practical form of a parametric amplifier employing an electron gunconstructed in accordance with the present invention is illustrated inFIGURE 3. The amplifier itself is described and claimed in the copendingapplication of Glen Wade, Serial No. 747,764, filed July 10, 1958,entitled Parametric Amplifiers, and assigned to the same assignee as thepresent invention. The tube assembly is disposed longitudinally withinan elongated envelope 40 through the opposite ends 41 and 42 of whichsuitable electrical connecting leads project. Disposed near end 42 is anelectron gun assembly 43. The electron gun includes tubular cathode 10supported by a ceramic wafer 45 from a metallic annulus 46 through whichan end of cathode 10 freely projects and on which end a cap is disposedand exteriorly coated with electron emissive material 11 which whenheated by a heater 48 emits electrons outwardly therefrom. Spaced behindcathode 10 is another metallic annulus 49 forming with annulus 46 a cagesubstantially confining cathode 10 except for the exposed coating 11.Spaced in front of emissive coating 11 is first anode 13 having itsaperture aligned with the axis of cathode 10 and beam path 12. Nextbeyond the metallic wafer forming anode 13 is accelerator anode 15 inthe form of a metal disc also having its aperture centered on beam path12 to accept and pass the electron beam. Successively spaced beyondaccelerator anode 15 are third and fourth anode electrodes 17 and 18likewise having their apertures centered on beam path 12.

Just beyond the electron gun is a modulator 50 for impressing signalenergy upon the electron beam. In the next portion of the beam pathoutwardly from the electron gun is a signal modulation expander 51 whichis followed by a demodulator 52 for removing signal energy from thebeam.

On beyond demodulator 52 is an electrode 53 having an aperture centeredon beam path 12 and which during operation serves as a suppressorelectrode. Finally, the

. a electrode beam is collected b anode 21' which is disposedtransversely of beam path 12 behind the aperture in suppressor 53. Theentire assembly is supported within envelope 40 by means of four ceramicrods 54 symmetrically disposed about beam path 12 and extending throughall of the various apertured electrodes and through suitable insulatingdiscs 55 in which the modulator, expander and demodulator electrodes aresecured. The diiferent electrodes are separated by suitable ceramicwashers 56 encircling ceramic rods 54 between the different electrodes;discs 55 are separated by similar ceramic sleeves 58. The entireassembly is held tightly together by means of compression Springs 59acting between a collector mounting plate 60 and a washer 61 pinned toceramic rods 54. Of course, suitable internal leads are brought out fromthe various electrodes to the terminals projecting through the basepresses.

An explanation of the operation of the modulator, expander anddemodulator is unnecessary for an understanding of the present inventionwhich is directed solely to the electron gun. Suffice it to say for thepresent that input signal energy impressed upon the beam in modulator 50develops waves representing the signal intelligenes and at the same timepreferably removes noise from the beam. In expander 51 the signal energyis parametrically amplified by subjecting the electrons to atime-variable inhomogeneous field. Demodulator 52 includes an electroncoupler, in this case identical with the electron coupler of modulator50, which derives the amplified signal energy from the beam.

In a successfully operated amplifier constructed as illustrated inFIGURE 3, electron gun emits electrons at a current density of about 100milliamperes per square centimeter, perhaps 99% of which is interceptedby first anode 13. With the anodes in the gun energized at potentials asdescribed above in the discussion of FIGURE 1, a beam is obtained havingcharacteristics resembling Brillouin flow. Interception of the beam bythe deflectors and receptors and other electrodes of the tube isnegligible. It may be noted in this respect that the electrodes in themodulator and demodulator define a space for beam travel approximately0.030 inch in width while the expander electrodes, of which there arefour in a quadrupole configuration, form a square 0.080 inch on a side.The lengths of the electron coupler and expander electrodes are 0.400inch. The apertures in support washers 55 have a diameter of 0.110 inchand the entire assembly is sealed within an envelope one inch indiameter. In actual practice, about six volts is applied to suppressor53 while collector 2.1 is biased at a potential of about 200 'volts, allvoltages discussed being positive with respect to the cathode.

While the electron gun has been described in connection with aparametric amplifier which operates within a magnetic field, it will bereadily apparent that the particular electron lens combination iscapable of finding specific utility in other electron beam devices.However, it has special significance in the production of a beam withina magnetic field. Since the gun generates a beam of accurately definedand constant cross-sectional area, it is most suitable for employment ina device wherein it is desired to pass the beam closely adjacent activeelectrodes while at the same time avoiding interception by theseelectrodes of any portion of the beam in order to avoid the introductionof noise into the signal channel as a result of such interception.

Devices such as certain parametric amplifiers depend for their operationupon the action of individual electrons within the beam. Most efiicientoperation is obtainable only if all electrons subjected to a given forcemove simultaneously in the same direction and with the same velocity asa result of that force. This is achieved through use of the presentinvention since all of the electrons initially have the same forwardvelocities.

The electron gun may be formed from simple, stamped i arts and itstolerances are similar to those of ordinary cathode-ray-tube guns; thatis, the tolerances are such as to be well within normal productiontechniques. Moreover, a fairly large latitude is offered in theserespects in that relative adjustment of the different potentials permitscompensation of production variations.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. An electron gun comprising: a cathode productive of an electronstream traveling along a predetermined axis and having a predeterminedcross-sectional area; a first anode, adjacent and positive with respectto said cathode, having an aperture on said axis and substantiallysmaller in cross-sectional area than said stream; a second anode, beyondand substantially positive with respect to said first anode, having anaperture on said axis; and third and fourth anodes, less positive thansaid second anode but positive with respect to said cathode, spacedsuccessively beyond said second anode and having apertures on said axisand of cross-sectional areas larger than that of said first anode.

2. An electron gun comprising: a cathode productive of an electronstream traveling along a predetermined axis and having predeterminedcross-sectional area; a first anode, adjacent and positive with respectto said cathode, having an aperture on said axis and substantiallysmaller in cross-sectional area than said stream; a second anode, beyondand substantially positive with respect to said first anode, having anaperture 'on said axis; third and fourth anodes, less positive than saidsecond anode but positive with respect to said cathode, spacedsuccessively beyond said second anode and having apertures on said axisand of cross-sectional areas larger than that of said first anode; andmeans fior subjecting said cathode and anodes to a magnetic field havingflux lines parallel to said axis.

3. An electron gun comprising: a cathode productive of an electronstream traveling along a predetermined axis and having a predeterminedcross-sectional area; a first anode, adjacent and positive with respectto. said cathode, having an aperture on said axis and substantiallysmaller in cross-sectional area than said stream; a second anode, beyondand substantially positive with respect to said first anode, having anaperture on said axis; third and fourth anodes, less positive than saidsecond anode but positive with respect to said cathode, spacedsuccessively beyond said second anode and having apertures on said axisand of cross-sectional areas larger than that of said first anode; andat least one additional anode, positive with respect to said cathode andspaced beyond said fourth anode, having an aperture on said axis and ofa cross-sectional area intermediate that of said first and fourthanodes.

4. An electron gun for projecting a beam of electrons of predeterminedconstant radius, along a predetermined axis and comprising: meansincluding a cathode for producing and accelerating an electron streamparallel to and including said axis and of predetermined crosssectionalarea; a convergent electrostatic lens efiectively located at andincluding a first anode adjacent and positive with respect to saidcathode and having an aperture on said axis and substantially smaller incross-sectional area than said stream; a second anode having an apertureon said axis and substantially positive with respect to and spacedbeyond said first anode; and a convergent electrostatic lens systemefiectively located at and including a third anode having an aperture onsaid axis of an area larger than that of said first anode aperture andsubstantially negative with respect to and spaced beyond said secondanode,

but positive with respect to said cathode.

5. An electron gun for projecting a beam of electrons of predeterminedconstant radius along a predetermined axis and comprising: meansincluding a cathode for producing and accelerating an electron streamparallel to and including said axis and of predetermined cross-sectionalarea and current density; a convergent electrostatic lens eflectivelylocated at and including a first anode adjacent and positive withrespect to said cathode and having an aperture on said axis and of across-sectional area substantially smaller than said predeterminedcross-sectional area; means, including a second anode having an apertureon said axis and spaced beyond and substantially positive with respectto said first anode, for permitting expansion of the envelope of saidbeam appnoximatelyto said predetermined radius and thereby substantiallyreducing said current density; and means,

including a convergent electrostatic lens system effectively' located atand including a third anode substantially negative with respect to saidsecond anode and having an aperture on said axis of an area larger thanthat of said first anode aperture and spaced beyond the second anode,for aligning said envelope parallel with said axis.

6. An electron gun for projecting a beam of electrons "of predeterminedconstant radius along a predetermined axis and comprising: meansincluding a cathode for producing and accelerating an electron streamparallel to and including said axis and of predetermined cross-sectionalarea and current density; a first convergent electrostatic lenseffectively located at and including a first anode adjacent and positivewith respect to said cathode and having an aperture on said axis and ofa cross-sectional area substantially smaller than said predetermined"cross-sectional area; means, including a second anode having anaperture on said axis with said second anode spaced beyond andsubstantially positive with respect to said first anode, for permittingexpansion of the envelope of said beam to said predetermined radius andthereby substantially reducing said current density; means, including aconvergent electrostatic lens system elfectively located at andincluding a third anode substantially negative with respect to saidsecond anode and spaced there- 7. An electron gun for projecting a beamof electrons of predetermined constant radius along a predetermined axisand comprising: means including a cathode for producing and acceleratingan electron stream parallel to and including said axis and ofpredetermined cross-sectional area and current density; a firstconvergent electrostatic lens efiectively located at and including afirst anode adjacent and positive with respect to said cathode andhaving an aperture on said axis and of a cross-sectional areasubstantially smaller than said predetermined cross sectional area;means, including a divergent electrostatic lens efiectively located atand comprising a second anode having an aperture on said axis with saidsecond anode spaced beyond and substantially positive with respect tosaid first anode, for permitting expansion of the envelope of said beamat least to said predetermined radius and thereby substantially reducingsaid current density; means, including a convergent electrostatic lenssystem effectively located at and including a third anode having anaperture on said axis larger in area than said first anode aperture andspaced beyond and substantially negative with respect to said secondanode, for aligning said envelope parallel with said axis; and means,including at least one additional anode spaced beyond said lens systemand having an aperture on said path with a radius equal to saidpredetermined radius, for intercepting only any outer circumferentialportion of said stream of a radius greater than said predeterminedradius.

8. An electron gun for projecting a beam of electrons, of predeterminedconstant radius, along a predetermined axis and comprising: meansincluding a cathode for pro- ,ducing and accelerating an electron streamparallel to and including said axis and of predetermined cross-sectionalarea; a convergent electrostatic lens efiectively located at andincluding a first anode adjacent and positive with respect to saidcathode and having'an aperture on said axis and substantially smallerthan said stream; a second anode having an aperture on said axis and ata positive potential with respect to and at least an order of magnianodehaving an aperture on said axis larger in area than said first anodeaperture.

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